Summary

XPA supports both inet and unix (local) socket communication.

Description

XPA uses sockets for communication between processes. It supports
three methods of socket communication: inet, localhost, and unix. In
general, the same method should be employed for all XPA processes in a
session and the global environment variable XPA_METHOD should be used
to set up the desired method. By default, the preferred method is
"inet", which is appropriate for most users. You can set up a
different method by typing something like:

The options for XPA_METHOD are: inet, unix (or
local), and localhost. On Unix machines, this
environment setup command can be placed in your shell init file
(.cshrc, .profile, .bashrc, etc.) On Windows platforms, it can be
placed in your AUTOEXEC.BAT file (I think!).

By default, inet sockets are used by XPA. These are the standard
Internet sockets that are used by programs such as Netscape,
ftp. etc. Inet sockets utilize the IP address of the given machine and
a (usually random) port number to communicate between processes on the
same machine or between different machines on the Internet. (Note that
XPA has an Access Control mechanism to
prevent unauthorized access of XPA access points by other computers on
the Net). For users connected to the Internet, this usually is the
appropriate communication method. For more information about setting
up XPA communication between machines, see
Communication Between Machines.

In you are using XPA on a machine without an Internet connection, then
inet sockets are not appropriate. In fact, an XPA process often will
hang for many seconds while waiting for a response from the Domain
Name Service (DNS) when using inet sockets. Instead of inet sockets,
users on Unix platforms can also use unix sockets (also known
as local sockets). These sockets are based on the local file system
and do not make use of the DNS. They generally are considered to be
faster than inet sockets, but they are not implemented under
Windows. Use local sockets as a first resort if you are on a Unix
machine that is not connected to the Internet.

Users not connected to the Internet also can use localhost
sockets. These are also inet-type sockets but the IP address used for
the local machine is the localhost address, 0x7F000001, instead
of the real IP of the machine. Depending on how sockets are set up for
a given platform, communication with the DNS usually is not required in
this case (though of course, XPA cannot interact with other machines).
The localhost method will generally work on both Unix and Windows
platforms, but whether the DNS is required or not is subject to
individual configurations.

A final warning/reminder: if your XPA-enabled server hangs at startup
time and your XPA_METHOD is inet, the problem probably is
related to an incorrect Internet configuration. This can be confirmed
by using the unix method or (usually) the localhost
method. You can use these alternate methods if other hosts do not need
access to the XPA server.